Although most automobile and truck engines (automotive vehicle engines) are operated on gasoline or diesel fuel, natural gas is recognized as a potential fuel for automotive vehicles because it is viewed as a "clean" fuel. Natural gas comprises mostly methane (CH.sub.4). It has been found that engines operated with methane or natural gas as a fuel produce lower amounts per mile of carbon monoxide, carbon dioxide and unburned hydrocarbons of the type that contribute to smog than engines operated on gasoline. The lower quantity of such hydrocarbon emissions is seen as particularly beneficial because of the corresponding reduction in the formation of ground level ozone. The reduction in carbon dioxide is also beneficial because carbon dioxide is a greenhouse effect gas. Since gasoline and natural gas are both hydrocarbon fuels, it would seem that operating practices and exhaust treatment techniques developed for gasoline engines would be directly applicable to methane-fueled engines. However, such is not the case.
Over the past 20 years, noble metal catalysts supported on high surface area (100 m.sup.2 /g) alumina carriers have been developed to complete the oxidation of carbon monoxide and unburned hydrocarbons in gasoline engine exhaust. Platinum and/or palladium dispersed as very fine particles on pellets or grains of alumina have served as oxidation catalysts These catalysts have proven most effective when there is an excess of oxygen in the exhaust gas resulting when the engine is operating in a fuel-lean or excess-air mode. The catalytic conversion of nitrogen oxides to nitrogen is a chemical reduction-type reaction which is most favorably carried out in an oxygen-deficient environment that is the antithesis of a favorable oxidation reaction medium. However, the noble metal rhodium has been successfully used in combination with platinum, palladium or platinum and palladium as a "three-way catalyst". Under suitable engine operating conditions, a three-way catalyst promotes simultaneously the oxidation of carbon monoxide to carbon dioxide, the oxidation of unburned hydrocarbons to carbon dioxide and water and the reduction of nitrogen oxides to nitrogen.
Three-way catalysts work most effectively when the engine is operated with the air-fuel mixture at about stoichiometric proportions. An oxygen sensor is used in the exhaust gas stream to detect whether the engine is then operating in a fuel-rich or fuel-lean mode. Output from the sensor is used by the engine control computer to continually effect rapid adjustments in the fuel-to-air mass ratio so that the combustible charge to the engine cycles close to the stoichiometric air-fuel mixture. The actual air-fuel ratio is thus sensed and changed as necessary so as to reduce engine out emissions and to provide a suitable feed stream to the engine exhaust treatment catalytic converter. By thus cycling the engine air-fuel ratio, a three-way catalyst is able to promote and support its three pollutant-destroying reactions more or less simultaneously.
This three-way catalyst practice represents the current state of the art in gasoline-fueled engine exhaust treatment. Three-way catalysts comprise a combination of platinum, palladium or platinum and palladium with a small amount of rhodium, all dispersed as extremely fine particles on a high surface area alumina (Al.sub.2 O.sub.3) carrier. The alumina is thermally stabilized in its high surface area form by the presence of suitable additives such as ceria, lanthana and others. Suitable additions of ceria (CeO.sub.2) may also promote the oxidation capacity of the catalyst when the exhaust is momentarily in an oxygen-deficient state.
The problem that has been discovered with natural gas (methane, CH.sub.4) fueled engines is that when operated with three-way catalysts in accordance with gasoline-fueled engine practices, unburned methane passes unoxidized through the exhaust system into the atmosphere. Although methane is not poisonous and it is not a reactive hydrocarbon in the sense that it promotes ozone formation at low altitudes, it is a greenhouse effect gas. It remains in the atmosphere and has many times the atmospheric heat-reflecting effect of carbon dioxide.
We have found that unlike higher molecular weight hydrocarbon gases, methane is not readily oxidized in an oxygen-rich exhaust gas over the traditional noble metal catalysts These catalysts do not become "active" to oxidize methane until heated to very high temperatures (e.g., 600.degree. C. or higher) which the exhaust gases usually do not attain Thus, while the engine out emissions with methane fuel are favorable compared to gasoline-fueled engines, there remains the problem of preventing unburned methane from escaping the vehicle's exhaust system into the atmosphere.
It has been observed in the operation of natural gas-fueled stationary engines, typically used in the generation of electrical energy and the concomitant production of process heat, that the conventional three-way catalysts will effect some conversion of each of methane, HC, CO and NOx if the engine is operated in a very narrowly controlled air-to-fuel ratio that is just fuel-rich of the stoichiometric mixture. However, it is necessary to have a better and more flexible catalyst system and engine operating system for natural gas-fueled automobile engines to prevent methane enrichment of the atmosphere. Unlike stationary engines, automobile engines experience warm-up operating conditions and other transient operating conditions of widely varying load and speed.
It is an object of this invention to provide a method of engine operation and exhaust gas treatment for natural gas-fueled automobile engines that provide good fuel economy and effectively reduce the quantity of carbon monoxide (CO), unburned hydrocarbons (HC) including methane (CH.sub.4), and nitrogen oxides (NOx) that are discharged to the atmosphere.